The present invention relates to equipment for fiber-optic communications networks, and more particularly, to optical amplifiers in which optical gain is provided by stimulated Raman scattering.
Fiber-optic networks are used to support voice and data communications. In optical networks that use wavelength division multiplexing, multiple wavelengths of light are used to support multiple communications channels on a single fiber.
Optical amplifiers such as Raman optical amplifiers and erbium-doped fiber amplifiers are used in fiber-optic networks to amplify optical signals. In a typical fiber-optic communications link, erbium-doped fiber amplifiers may be located at network nodes between successive spans of transmission fiber. The erbium-doped fiber amplifier at each node boosts the signal power of the data signals traveling along the link.
Distributed Raman amplifiers may be formed by providing backwards-propagating Raman pump light to the spans of transmission fiber. The Raman pump light produces Raman gain in the transmission fiber through stimulated Raman scattering. The Raman gain amplifies the signals traveling along the span before the signals reach the erbium-doped fiber amplifiers at the nodes.
It is an object of the present invention to provide improved Raman amplifier equipment for fiber-optic communications networks.
This and other objects of the invention are accomplished in accordance with the present invention by providing Raman optical amplifier equipment that creates Raman amplification for optical data signals being carried on fiber links in fiber-optic communications networks. Distributed Raman amplifiers and discrete Raman amplifiers may be used to provide Raman amplification. The Raman amplifiers may be used in conjunction with other types of amplifiers such as rare-earth-doped fiber amplifiers or semiconductor optical amplifiers.
Raman amplification may be provided by pumping transmission fiber spans or coils of fiber using a cascaded Raman amplifier approach. Pump light at a first wavelength or band of wavelengths may be used to generate Raman gain for light at a second wavelength or band of wavelengths. The light at the second wavelength or wavelengths may be used to produce Raman gain in the signal band. This approach may be used to extend the Raman gain in the signal band farther into the Raman-pumped fiber than would be possible without the assistance of the pump light at the first wavelength or wavelengths. Moving the Raman gain farther into the fiber may help to improve the noise figure of the amplifier.
Raman gain transients may be controlled by monitoring fluctuations in the signal light and adjusting the Raman pump power accordingly. For example, the total power of the signals on a link or the power of the signals being carried on a particular channel or channels such as a telemetry channel may be monitored. The power of the Raman pump light may be adjusted in real time based on these signal measurements to ensure that the Raman gain in the signal band remains at a desired level.